The present invention relates to Zircaloy alloy intermediate and final products, and processes for their fabrication. More particularly, this invention is especially concerned with Zircaloy alloys having a particular microstructure, and the method of producing this microstructure, such that the material has improved long term corrosion resistance in a high temperature steam environment.
The Zircaloy alloys were initially developed as cladding materials for nuclear components used within a high temperature pressurized water reactor environment (U.S. Pat. No. 2,772,964). The Zircaloy-2 alloy is an alloy of zirconium comprising about 1.2 to 1.7 weight percent tin, about 0.07 to 0.20 weight percent iron, about 0.05 to 0.15 weight percent chromium, and about 0.03 to 0.08 weight percent nickel. The Zircaloy-4 alloy is an alloy of zirconium comprising about 1.2 to 1.7 weight percent tin, about 0.12 to 0.18 weight percent iron, and about 0.05 to 0.15 weight percent chromium (see U.S. Pat. No. 3,148,055).
In addition variations upon these alloys have been made by varying the above listed alloying elements and/or the addition of amounts of other elements. For example in some cases it may be desirable to add silicon to the Zircaloy-2 alloy composition as taught in U.S. Pat. No. 3,097,094. In addition oxygen is sometimes considered as an alloying element rather than an impurity, since it is a solid solution strengthener of zirconium.
Nuclear grade Zircaloy-2 or Zircaloy-4 alloys are made by repeated vacuum consumable electrode melting to produce a final ingot having a diameter typically between about 16 and 25 inches. The ingot is then conditioned to remove surface contamination, heated into the beta, alpha+beta phase or high temperature alpha phase and then worked to some intermediate sized and shaped billet. This primary ingot breakdown may be performed by forging, rolling, extruding or combinations of these methods. The intermediate billet is then beta solution treated by heating above the alpha+beta/beta transus temperature and then held in the beta phase for a specified period of time and then quenched in water. After this step it is further thermomechanically worked to a final desired shape at a temperature typically below the alpha/alpha+beta transus temperature.
For Zircaloy material that is to be used as tubular cladding for fuel pellets, the intermediate billet may be beta treated by heating to approximately 1050.degree. C. and subsequently water quenched to a temperature below the alpha+beta to alpha transus temperature.
Depending upon the size and shape of the intermediate product at this stage of fabrication, the billet may first be alpha worked by heating it to about 750.degree. C. and then forging the hot billet to a size and shape appropriate for extrusion. Once it has attained the desired size and shape (substantially round cross-section), the billet is prepared for extrusion. This preparation includes drilling an axial hole along the center line of the billet, machining the outside diameter to desired dimensions, and applying a suitable lubricant to the surfaces of the billet. The billet diameter is then reduced by extrusion at 700.degree. C. or greater through a frustoconical die and over a mandrel. The as-extruded cylinder may then be optionally annealed at about 700.degree. C. Before leaving the primary fabricator the extruded billet may be cold worked by pilgering to further reduce its wall thickness and outside diameter. At this stage the intermediate product is known as a TREX (Tube Reduced Extrusion). The extrusion or TREX may then be sent to a tube mill for fabrication into the final product.
At the tube mill the extrusion or TREX goes through several cold pilger steps with anneals at about 675.degree.-700.degree. C. between each reduction step. After the final cold pilger step the material is given a final anneal which may be a full recrystallization anneal, partial recrystallization anneal, or stress relief anneal. The anneal may be performed at a temperature as high as 675.degree.-700.degree. C. Other tube forming methods such as sinking, rocking and drawing may also completely or partially substitute for the pilgering method.
Thin-walled members of Zircaloy-2 and Zircaloy-4 alloys, such as nuclear fuel cladding, processed by the above-described technique, have a resultant structure which is essentially single phase alpha with intermetallic particles (i.e. precipitates) containing Zr, Fe, and Cr, and including Ni in the Zircaloy-2 alloy. The precipitates for the most part are randomly distributed, through the alpha phase matrix, but bands or "stringers" of precipitates are frequently observed. The larger precipitates are approximately 1 micron in diameter and the average particle size is approximately 0.3 microns (3000 angstroms) in diameter.
In addition, these members exhibit a strong anisotropy in their crystallographic texture which tends to preferentially align hydrides produced during exposure to high temperature and pressure steam in a circumferential direction in the alpha matrix and helps to provide the required creep and tensile properties in the circumferential direction.
The alpha matrix itself may be characterized by a heavily cold worked or dislocated structure, a partially recrystallized structure or a full recrystallized structure, depending upon the type of final anneal given the material.
Where final material of a rectangular cross section is desired, the intermediate billet may be processed substantially as described above, with the exception that the reductions after the beta solution treating process are typically performed by hot, warm and/or cold rolling the material at a temperature within the alpha phase or just above the alpha to alpha+beta transus temperature. Alpha phase hot forging may also be performed. Examples of such processing techniques are described in U.S. Pat. No. 3,645,800.
It has been reported that various properties of Zircaloy alloy components can be improved if beta treating is performed on the final size product or near final size product, in addition to the aforementioned conventional beta treatment that occurs early in the processing. Examples of such reports are as follows: U.S. Pat. No. 3,865,635 and U.S. Pat. No. 4,238,251. Included among these reports is the report that good Zircaloy-4 alloy corrosion properties in high temperature steam environments can be achieved by retention of at least a substantial portion of the precipitate distribution in two dimensional arrays, especially in the alpha phase grain boundaries of the beta treated microstructure. This configuration of precipitates is quite distinct from the substantially random array of precipitates normally observed in alpha worked (i.e. below approximately 1450.degree. F.) Zircaloy final product where the beta treatment, if any, occurred much earlier in the breakdown of the ingot as described above. The extensive alpha working of the material after the usual beta treatment serves to break up the two dimensional arrays of precipitates and distribute them in the random fashion typically observed in alpha-worked final product.
It has been found that conventionally processed, alpha worked Zircaloy cladding (tubing) and channels (plate) when exposed to high temperature steam such as that found in a BWR (Boiling Water Reactor) or about 450.degree. to 500.degree. C., 1500 psi steam autoclave test have a propensity to form thick oxide films with white nodules of spalling corrosion product, rather than the desirable thin continuous, and adherent, substantially black corrosion product needed for long term reactor operation.
According to the present invention it has surprisingly been found that small modifications made to conventional processing of Zircaloy alloys provide significantly improved high temperature steam corrosion resistance while maintaining the substantially random distribution of precipitates and strong anistropic texture observed in conventionally alpha worked Zircaloy final product. It is believed that if beta treating is performed at or near the conventional early stage of processing, that limiting the elevation of the subsequent alpha working and annealing temperatures, and/or the time at temperature, will provide final product with the required corrosion resistance. It has been observed in final product according to the present invention that the average precipitate size has been reduced substantially below that observed in conventionally processed alpha worked product. It has been found that if these subsequent alpha working and annealing temperatures are limited to below approximately 600.degree. to 625.degree. C. that intermediate and final product having precipitate sizes significantly below that observed in conventionally processed Zircaloy are produced. Articles, thus processed have been found to exhibit significantly lower corrosion weight gains in comparison with conventionally processed material in high temperature steam tests.
These and other aspects of the invention will become more apparent upon review of the drawings in conjunction with the detailed description of the invention.